As Rupert Holmes notes in the script, Neville and Helena Landless are supposed to be from Ceylon (modern day Sri Lanka) but do not need to actually be played by actors of that ethnicity. In the context of the show, Neville and Helena are played by Victor Grinstead and Janet Conover, who are members of the company of the Music Hall Royale and would have been, assumably, English themselves. That's not to say that they have to be white, but it doesn't mean that they have to look like they're actually from Ceylon. It's supposed to be a British acting troupe's representation of what they though the people of Ceylon were like (ie, the characterizations are typically pretty over-the-top), so Karl and Mueller are fine casting. It's really a great company as a whole, I think. My one curiosity is which version of the script they'll be using.
"Art, in itself, is an attempt to bring order out of chaos."-Stephen Sondheim